An Ocean Thermal Energy Conversion (OTEC) system generates electrical energy based on a naturally occurring temperature difference between water at the surface of a large body of water and water thousands of meters deep. As long as the temperature between the warm surface water and the cold deep water differs by about 20° C., an OTEC system can produce a significant amount of power. Oceans (and other large bodies of water, such as seas, large lakes, etc.), therefore, represent vast renewable energy resources, which can be relatively easy to access.
Conventional OTEC systems are typically located far offshore on an offshore platform, such as a tension-leg platform, semi-submersible, spar, drill ship, jack-up offshore platform, grazing plant, and the like. An OTEC electrical generation system normally generates electrical energy by driving a turbogenerator with a working fluid, such as ammonia, that circulates through the OTEC system in a closed loop that includes the turbogenerator, an evaporator, and a condenser.
In operation, heat from warm seawater taken from the surface of the ocean is absorbed by liquid working fluid at the evaporator causing the working fluid to vaporize and expand. The expanding vapor is forced through the turbogenerator, which, in turn, generates electrical energy. After passing through the turbogenerator, the working fluid enters the condenser where its heat is absorbed by cold seawater pumped from a deep-water region of the ocean. As a result, the vaporized working fluid to return to its liquid form. The liquefied working fluid is then pumped back to the evaporator to being the cycle anew.
The evaporator and condenser include suitable configured heat exchangers, at which heat is transferred between the working fluid and seawater. These heat exchangers must be able to withstand prolonged exposure to relatively corrosive working fluid, as well as a large secondary flow of the seawater. In addition, it is preferable, if not required, that such heat exchangers provide high overall heat transfer coefficients, exhibit minimal mechanical pumping losses, and are lightweight. Further, the heat exchangers represent a significant portion of the overall OTEC system cost; therefore, it is important that their materials and fabrication costs are not excessive.
In a typical OTEC configuration, the evaporator and condenser heat exchangers are located on the deck of the offshore platform so they are readily accessible for service and maintenance. Recently, however, OTEC systems have been developed wherein the heat exchangers are located underwater, mounted to the platform support structure. This can, among other things, reduce platform cost, preserve deck space, increase system efficiency, and reduce the complexity of the flow system necessary to bring seawater into and out of the heat exchangers.
Unfortunately, OTEC heat exchangers are highly susceptible to biofouling, corrosion, and degradation over the operating lifetime of their system—particularly when mounted in a submerged location. It is vital, therefore, that they are accessible for regular maintenance, emergency repair, and/or replacement. In some cases, submerged heat exchangers are housed in underwater compartments located within (or attached to) the platform support structure itself. This provides some of the benefits of heat exchanger submersion but also keeps them relatively available for service. Unfortunately, these submerged compartments can add significant cost, size, and complexity to the offshore platform.
In addition, it is sometimes necessary to upgrade the capability of an OTEC system by, for example, adding or upgrading one or more heat exchangers to increase heat-transfer capacity, etc. Preferably, the OTEC system should be capable of continuous operation during maintenance and upgrade procedures. Removal and/or attachment of a submerged heat exchanger can be extremely challenging, however—especially in cases when such operations require personnel to gain access to underwater compartments and/or require special diver operations. As a result, it is often necessary to shut down the entire OTEC system during maintenance, repair, and upgrade operations, which can significantly impact overall production capability.